ABSTRACT
The kidney of the teleost is a mixed organ comprising hematopoietic, phagocytic, endocrine, and excretory elements. The component structure of the fish nephron varies considerably between marine, euryhaline, and freshwater. A typical freshwater nephron consists of cytologically distinct regions: renal corpuscles (RCs), neck segment, proximal, intermediate, and distal tubules (DTs). The fish RCs consist of a glomerulus and Bowman’s capsule. The neck region is continuous with the parietal and visceral epithelia of Bowman’s capsule and shows a narrow lumen surrounded by ciliated epithelial cells. The proximal convoluted tubule is lined by eosinophilic-granular columnar cells with a well-developed brush border. The intermediate segment has a narrow lumen surrounded by cuboidal cells that often have cilia. Within the DTs, more water is resorbed and urine concentrated or diluted. Collecting tubules and ducts are located throughout the kidney and are involved in the collection of concentrate for excretion and more water resorption. The ureters, which conduct the urine from the collecting ducts to the urinary papilla, may fuse at any level and may be dilated, after fusion, to form a bladder. The bladder can be only a simple dilation of the ureters or a true saccular organ emptying outside by a urogenital pore.
The fish urinary system is composed of the kidney, extrarenal urinary ducts, and the urinary bladder (UB). The kidney of the teleost is a mixed organ comprising hematopoietic, phagocytic, endocrine, and excretory elements. The kidneys vary greatly between different species of fish, both grossly and histologically, often (partially or totally) fused (Clupeidae, Salmonidae, Anguillidae, Cyprinidae, etc.). The kidney of fish is usually located in a retroperitoneal position up against the ventral aspect of the vertebral column. It is a light or dark brown or black organ normally extending the length of the body cavity. The entire kidney of adult fishes is a mesonephros type. It is usually divided into anterior or head kidney, which is largely composed of hematopoietic elements, which also contains chromaffin and 238adrenocortical endocrine elements; few renal tubules (RTs) are observed. The posterior kidney (Figure 11.1A–C) contains more RTs with a lesser amount of interstitial hematopoietic and lymphoid tissues and thus functions as an osmoregulatory organ. The ureters, which conduct urine from the collecting ducts to the urinary papilla, may fuse at any level and may be dilated, after fusion, to form a bladder. The urinary ducts open to the outside posterior to the anus.
